In a Long Term Evolution (LTE) LTE/LTE-Advanced (LTE-A) system of the 3rd Generation Partnership Project (3GPP), Orthogonal Frequency Division Multiple Access (OFDMA) is used as a downlink multiple access mode, and therefore a downlink resource of the system is divided into OFDM symbols in terms of time, and is divided into subcarriers in terms of frequency. Each OFDM symbol has a Cyclic Prefix (CP). There are types of CPs: normal CP (normal CP) and extended CP (extended CP). When abase station schedules, in a downlink subframe, User Equipment (UE), that is, the base station needs to transmit, in the downlink subframe, downlink data to the UE, the minimum unit of a physical resource allocated to each UE is a Physical Resource Block (PRB). One PRB includes 12 subcarriers in a frequency domain, and is of half a subframe duration (that is, one timeslot) in a time domain, that is, includes seven OFDM symbols (if the normal CP is used) or six OFDM symbols (in a case in which the extended CP is used). One physical resource block pair includes 12 subcarriers in the frequency domain, and is of one subframe duration (that is, two timeslots) in the time domain.
Currently, in open-loop Multiple Input Multiple Output (MIMO) in the LTE R8, a maximum of four transmit antennas are supported. Rank-2, 3, or 4 transmission is supported. In open-loop MIMO, a Pre-coding Matrix Indicator (PMI) of a pre-coding matrix is not fed back; and when a rank (rank) is greater than or equal to 2, two channel coding codewords (codeword) are used to perform sending when a base station sends data. Four transmit antennas are used as an example; when sending data to UE, a base station performs, in a PRB pair that is scheduled to the UE, pre-coding by fixedly using, according to a rule, four pre-coding matrices of index numbers 12 to 15 rotationally. During feedback, Channel Quality Information (CQI) of only one codeword is fed back. In an open-loop mode, the reason that one piece of CQI may be fed back is that, in the open-loop MIMO mode, it is equivalent that layer shifting exists on adjacent subcarriers for two codewords, so that the two codewords pass through an approximately same channel, and therefore channel quality is also approximately the same. Therefore, CQI of one codeword may be used to express CQI of two codewords. For example, in rank-2 transmission (layers=2), on subcarriers n and n+1, pre-coding is performed by using a pre-coding matrix of an index 12. On the subcarrier n, codeword 1 is mapped to a first layer, codeword 2 is mapped to a second layer; however, on the subcarrier n+1, codeword 1 is mapped to the second layer, and codeword 1 is mapped to the first layer. Channels for adjacent subcarriers are basically the same, and therefore it may be considered that the two codewords pass through a same channel on the two subcarriers, and channel quality is also the same; therefore CQI of the two codewords may be expressed by using CQI of one codeword.
Currently, in LTE Rel.8/9/10/11, sending by using one, two, four, or eight antennas is supported in downlink transmission. In the closed-loop MIMO mode, Channel State Information (CSI) that User Equipment (UE) needs to feed back includes a rank indicator (rank indicator, RI), a PMI, CQI, and the like, where the RI and the PMI respectively indicate a layer quantity and a pre-coding matrix that the UE expects a base station to use to perform downlink data transmission. When the RI is greater than or equal to 2, two channel coding codewords (codeword) are used to perform sending when a base station sends data; in this case, an expression manner for CSI feedback by UE needs to include one PMI value, and CQI of two codewords. In the closed-loop MIMO mode, two codewords are mapped on a same subcarrier to different layers respectively; and a relationship of mapping, on all subcarriers, of either codeword to a layer, is the same, that is, no layer shifting exists. The two codewords pass through different channels; therefore CQI of the two codewords is independent from each other, and CQI of one codeword cannot be used to express the CQI of the two codewords. Therefore, the UE needs to separately feed back the CQI of the two codewords.
However, in the existing closed-loop MIMO mode, the expression manner for CSI feedback by UE is applicable if a quantity of configured antennas does not exceed eight; and when a quantity of downlink transmit antennas increases, for example, the quantity of downlink transmit antennas is 16, 32, or 64, no corresponding CSI feedback solution is provided in the prior art.